From WO 2005/044529, a device of such kind is known. The forming box of the apparatus described therein comprises a conveyer in the form of an endless belt screen adjacent to the row of spike rollers. As fibres enter the forming box they are sucked towards the forming wire and are distributed by the spike rollers, ensuring a relatively even distribution of fibrous material in the dry-formed mat. The conveyer ensures that no large lumps of fibrous material end up at the forming wire.